1. Field of the Invention
The present invention relates in general to compressing an audio signal for compensating for the effects of noise masking in a vehicle, and more specifically, for controlling the dynamic range of an audio reproduction in an interior of a vehicle, using variable parallel compression to the dynamics of the music.
2. Description of the Related Art
Audio systems within a vehicle are used to reproduce audio from a multimedia source for the occupants of the vehicle. The reproduced audio that is provided is dynamic in nature. That is, the volume level of the reproduced audio is constantly changing. Various portions of the reproduce audio are loud while other portions are soft, and such portions continuously follow each other. Unfortunately, background noise levels are generated within the vehicle, which drown out or mask portions of the reproduced audio. Background noise levels in vehicles are related to conditions such as speed, wind, engine, exhaust, and vibrations. Depending on the noise level of the background noise, lower or soft passages may be masked. Typically, engine idle speeds and exhaust systems tend to mask the low frequency levels while high engine rpm and wind noise tend to mask other frequency levels. As the background noise levels are increased, more passages of the reproduced audio are masked. Volume levels of the reproduced audio may be increased overall to bring the masked passages above the background noise level to make the masked passages audible over the background noise, however, this also increases the loud passages. By increasing the loud passages, the loud passages become too loud. If loud passages are limited, the reproduced audio loses its dynamic range.
Compression techniques have been used to increase the low-level portions of the audio signal. Conventional compression techniques rely on several variables to control the amount of compression and focus on squashing peaks in order to obtain a compressed audio signal. Adjustment of certain variables such as release times, rate of compression, onset of compression may work well for some audible signals but not well for others. Furthermore, it is difficult to correlate these variables to a single noise-related parameter such as speed, noise, or vibration. A conventional compression technique could be used that compresses the audio signal so that the high and low level portions of the signal are approximately the same volume. However, while this allows the low level portions, the dynamics of the audible signal is removed since the high level portions are at the approximate same volume level as the low level portions.